1. Field of the Invention
The present invention is related to a noise cancellation circuit for a mixer device and related mixer device, and more particularly to a noise cancellation circuit and related mixer device which can greatly reduce the flicker noise and increase the quality of the received information.
2. Description of the Prior Art
The mixer is a critical component in a wireless communication system, which is used for frequency translation which includes moving signals upward or downward in the frequency domain. According to the prior art, the mixer using the direct conversion architecture has the benefits of lower cost, lower energy consumption and higher degree of integration, and the Gilbert double-balanced mixer is one of the frequently used examples.
Please refer to FIG. 1, which illustrates a Gilbert double-balanced mixer 10. The Gilbert double-balanced mixer 10 comprises an RF (radio-frequency) input stage circuit 100, a mixer circuit 102 and an output stage circuit 104. The RF input stage circuit 100 comprises the field effect transistors M1 and M2, whose gates are used for receiving the differential RF signal VRF+ and VRF− from the antenna or the low noise amplifier (LNA), and the drains of the transistors are used for generating the corresponding differential current IRF+ and IRF−. The mixer circuit 102 comprises the field effect transistors M3, M4, M5 and M6, and is used for performing the function of frequency-mixing. The working principles behind the frequency-mixing process is to control the gains of the field effect transistors M3, M4, M5 and M6 by varying the magnitude of the currents IRF+ and IRF−, which are originated from drains of the transistors M1 and M2. Therefore, when the differential LO signal VLO+ and VLO− varies, the mixer circuit 102 will output the differential currents IIF+ and IIF− to the output stage circuit 104, which comprises the multiplication result of the gate voltages VLO+ and VLO− (of transistors M3-M6) and the drain current IRF+ and IRF− (of transistors M1-M2). Finally, the output stage circuit 104 comprises the resistors R1 and R2, which transfers the differential current IIF+ and IIF− to a differential low-frequency signal VIF+ and VIF−. In other words, the differential low frequency signal VIF+ and VIF− can be considered as the multiplication result of the differential RF signal (VRF+ and VRF−), and the differential LO signal (VLO+ and VLO−).
Inside the Gilbert double-balanced mixer 10, the four field effect transistor (M3-M6) of the mixer circuit 102 can generate the flicker noise owing to some of their intrinsic characteristics and the parasitic capacitance associated with the source of the transistors (M3-M6). The emergence of the flicker noise will greatly reduce the signal-to-noise ratio of the receiver circuit, and degrades the system performance. The frequency spectrum of the flicker noise overlaps with the frequency spectrum of the real signal, such that the flicker noise can't be removed simply by some frequency selective filtering process. Meanwhile, the frequency response of the flicker noise is approximately inversely proportional to the frequency; the energy of the flicker noise will get larger as the frequency goes lower. In other words, the real signals with some of their frequency components located close to the dc (direct current) will be seriously affected by the flicker noise. Therefore, for the purpose of increasing the quality of the received signals and decreasing the error rate of the received information, it is a must for the receiver circuit to decrease the influence of the flicker noise to the real signals.